The present invention relates to rolling seamless hollow stock for purposes of making seamless tubes by means of diagonal rolling, using frustoconical rolls.
German Pat. No. 174 372 disclosed a mill, using two or more obliquely oriented rolls of frustoconical or truncated-cone-like configuration. These rolls are arranged in a common plane of rolling, and they are oriented so that the thick end of each roll is located downstream as far as the movement of the rolled stock is concerned. The oblique orientation follows particular rules; its angle with reference to a first plane, that includes the axis of rolling; establishes the transport angle, while the angle relative to a second plane, which includes also the axis of rolling and is oriented orthogonally to the first plane, is the spreading angle, being approximately half the apex angle of the frustocone. The frustoconical surface of the roll can be subdivided into a concial feed or entrance portion, thicker and thinner portions (with reference to an ideal cone) and an exit or discharge portion at the thicker end of the cone. These rolls cooperate with a piercing mandrel.
Pipes made in that manner are usually subsequently sized, also by diagonal rolls, and they are further stretched. Sizing and stretching is also used for making tubes from hollows which have been produced initially otherwise. The hollow blooms that are to be made are destined, e.g., for final wall thickness-to-diameter ratios of 1:15, or even thicker pipes, possibly being hollow blanks to be subsequently sized in pilger mills (reciporcating step rolling), or in a continuously working sizing mill, or stretching in a push-bank.
It has been suggested to make thin-walled, seamless pipes by stretching hollows, e.g., by means of multipass or multistand rolling, using rather small stretch values per pass or per stand and using diagonal rolls, possibly with a sizing shoulder. All of these proposals have not been realized in practice. It is believed that the thermal conditions interfer technologically with the desired goal of stretching. See, for example, German Pat. No. 926 541 or German printed patent application No. 960 328.